Our project focuses on the biology, cell biology, molecular biology and biochemistry of activated lymphocytes, cells that are used for immunotherapy, especially for cancer treatment, and involved in the cellular immune response to xenogeneic tissues. In an ongoing study of how aging affects the immune system using cytolytic T lymphocytes as a model, we have shown that an important signaling component of the T cell antigen receptor, namely the zeta chain, which is also important for signaling on NK cells, exhibits reduced gene expression with advancing age. In order to determine whether this change impacts the age-related decrease on T cell and NK functions in vitro and in vivo, and whether the effect can be reversed by genetic manipulation, we have bred, and are now aging, mice transgenic for the zeta chain. In our study on the regulation of human natural killer (NK) and IL-2-activated NK (lymphokine-activated killer or LAK cells), we have expanded our earlier findings that signaling through Fas or cell surface adhesion molecules can induce apoptosis of IL-2-activated but not native NK cells to show that the Fas-mediated activation-induced cell death of NK cells is regulated through an oxidation-reduction sensitive pathway involving phosphatase activities. Moreover, we have found that human NK cells express endothelial nitric oxide (NO) synthase, and that NO protects them from activation induced cell death through a mechanism involving activation of the transcription factors, NFAT. We have made and explored the observation that FasL expression is also regulated by redox, and have obtained data to suggest that oxidative stress supresses FasL induction through suppression of a calciuneurin-NFAT-dependent pathway. Finally, we have shown that cytokine-activated NK cells produce interleukin-10, suggesting a potential feedback regulatory loop. We have expanded our studies of xenogeneic immune responses in the human anti-pig (host vs. graft) and pig anti-human (graft vs host) responses in vitro. Our data show that NK cells are the primary mediators of the human vs. pig response, and that this response is modulated by several human cytokines, and by redox conditions.